Frequency-division duplex (FDD) and many other radio types include both a transmitter and receiver, often operating simultaneously on different frequency bands. Leakage and intermodulation (IM) effects can cause unwanted artifacts and noise from the transmitted signal to be introduced into the receiver signal path. This adds noise to the received signal, decreasing the dynamic range and sensitivity of the receiver, and reduces overall receiver performance.
Leakage of a transmit signal into the receive path generally causes two problems. The first is generation of new IM noise in any of the nonlinear devices in the receive path. Some of this will show up as in-band (i.e. within the receive band) noise, thereby worsening the signal-to-noise ratio and making it hard or impossible to receive signals from weaker, desired stations. The second problem is that the amplitude of a leaked transmit signal at an input of an analog-to-digital converter (ADC), even if its frequency doesn't overlap the desired receive signal, may be much stronger than the amplitude of that desired receive signal. To prevent clipping of the ADC, several (most significant) bits of resolution may need to be sacrificed, reducing the resolution with which the desired receive signal can be digitized, and increasing digitization noise.
One prior approach to reducing leakage and intermodulation noise is to include a guard band between the transmitter and receiver frequency bands. Most often, some noise will still leak past the guard band into the receiver frequency band. Increasing the guard band reduces the amount of leakage noise, but may waste overall bandwidth.
FIG. 1a illustrates an example system 100 with two communicating frequency-division duplex (FDD) radios: Radio-A 110 and Radio-B 150. Each radio contains an associated transmitter 111, 115 and receiver 114, 112, respectively. In this example, the signal transmitted by Radio-A 110 and received by Radio-B 130 is called the downlink (DL) signal 120. Similarly, the signal transmitted by Radio-B 130 and received by Radio-A 110 is called the uplink (UL) signal 140. One illustrative context includes Radio-A 110 in a base station and Radio-B 130 in a mobile device such as a mobile phone.
FIG. 1b illustrates an example frequency spectrum 150 in an FDD system such as drawn in FIG. 1a. The DL signal band 160 and UL signal band 180 occupy disjoint but not necessarily contiguous frequency bands. Generally, there is also an inactive or unused portion of the spectrum between the DL band 160 and UL band 180 called the guard band (GB) 170. Without loss of generality, FIG. 1b illustrates the DL band 160 as being lower in frequency than the UL band 180, but those skilled in the art understand that this need not be the case in general.
The problems caused by leakage and intermodulation are common in communication systems, whether using FDD and GBs or not. Examples include multi-user access methods, including but not limited to using time-division multiplexing (TDD) or Orthogonal Frequency Division Multiple Access (OFDMA), its derivatives such as Discrete Fourier Transform (DFT) pre-coded OFDMA or code-division multiple access (CDMA). Furthermore, in some systems, instance frequency bands may be fragmented, rather than show the simple spectrum stacking of FIG. 1b. 
Communication systems may also use space division and diversity duplexing in combination with frequency division duplexing. Such systems are subject to large near-far effects that limit system performance by requiring a large dynamic range for signals in the receive path of the station. For instance, a base station, while transmitting its own powerful downlink to multiple mobile devices, may simultaneously receive a relatively strong signal from a nearby mobile device and a relatively weak signal from a far away mobile device.
While the problems caused by leakage and intermodulation may have a different scale in a base station than in a mobile device, they do not have a different nature. Therefore, this document will further describe FDD radios without distinction as to their place in the overall communication system.
A large source of distortion in the transmit path is the power amplifier. It is often operated in its nonlinear region in order to improve the power transfer efficiency of the system. FIG. 3 shows an example power transfer diagram 300 plotting the output power 320 of a power amplifier versus its input power 310. The power amplifier is characterized by curve 340 which is linear for a limited region of operation. When the power amplifier is operated beyond its linear region, peak power is not delivered as needed, and the resulting waveform distortions show up as third and higher order effects in the frequency spectrum, leaking outside the desired frequency band of the transmit signal.
There is an unmet need to reduce the presence of noise caused by a transmit signal and other sources at all stages of the receive path.